The present invention relates to a passive restraint for an automobile, and more particularly, to a knee blocker for automotive applications that is positioned under the steering column forward of the knees.
Recently, the National Highway Traffic Safety Administration promulgated standard No. 208 entitled Occupant Crash Protection which is applicable to all automobiles manufactured by 1990 and requires that the impinging load upon the knee and into the femur or thigh area during an automobile crash should not be more than 2250 pounds per leg. If the driver does not wear a lap belt, the forward motion of the driver into the instrument panel creates a femur load in both legs vastly greater than 2200 lbs. As a result thereof, the automobile industry has actively been pursuing various designs for passive restraints which absorb the impact when the knees are forced into the instrument panel to create an impinging load on the femur or thigh area during an automobile crash of less than 2250 pounds per leg. Such a design is the subject of U.S. Pat. No. 4,721,329 to Brantman et al. issued Jan. 26, 1988.
Typically, these passive restraints include a suitable energy-absorption material that is enveloped in a cavity of the passive restraint. In the Brantman reference, an elastic polyethylene foam layer having a density of about 6 lbs/ft.sup.3, and about 11/2 inches thick absorbs energy upon impact of the knees. In addition, in Brantman, to provide further energy absorption an outer layer is formed of crushable polystyrene foam about 11/2 inches thick, and having a density 2 lbs/ft.sup.3. It has been found extremely expensive, however, during the manufacturing process to produce a passive restraint having a layer of crushable pellets and a layer of polyethylene foam material as is disclosed in Brantman.
In order to alleviate this cost, the present invention includes a passive restraint defining a frame having an internal cavity including microsphere-filled thermoset resin pellets which are extruded in paste form, gelled and pelletized. These thermoset resin pellets are the subject of U.S. Pat. No. 4,737,407 issued Apr. 12, 1988 to Joseph Wycech, the subject matter of which is incorporated by reference. In contrast to the Brantman reference, the necessity of a polyethylene foam material being intermixed with crushable pellets in an internal cavity of the passive restraint is alleviated by this invention.
As further background, plastic materials are currently used for filling and reinforcing structural members. Expanded polyurethane foam is known to be used for filling structural members to improve sound dampening, thermal insulating and crush strength qualities of the structures. Plastic fillings are used in boats to fill floatation cavities and in vehicles to act as sound baffles and reinforcements for hollow structural members.
The most common type of plastic used in such applications is expanded polyurethane foam. In structural reinforcement applications, expanded polyurethane foam lacks compressive and tensile strength and has extremely low heat resistance.
In recent years, specialized plastic reinforcements have been developed wherein macrospheres are formed of glass microspheres which are combined with a phenolic binder. The macrospheres are then coated with a phenolic resin which increases the strength and shell thickness of the macrospheres but also adds weight to the final product. After coating with a phenolic resin, the macrospheres are coated with a B-staged phenolic or epoxy which permits the macrospheres to be bound together to form a structural reinforcement. Examples of two types of such macrospheres are two materials manufactured by 3M Company and identified by the following trade designations: M27X for uncoated macrospheres and M35EX for phenolic-coated macrospheres. The above macrospheres are known to be used as structural reinforcements for vehicles.
Another approach to improving the performance of plastic fillers to function as reinforcements is to provide styrofoam beads which are coated with an epoxy. One such product is sold by W. D. Grace Company under the tradename Ecosphere. The styrofoam bead has an epoxy coating which is in the form of a cured shell. The styrofoam bead with cured shell may be coated with an adhesive and used as a constituent element for structural fillers. However, the coated styrofoam beads are expensive and have only slightly greater compressive strength than polyurethane fillers and have only limited heat resistance due to the fact that the styrofoam substrate may begin to melt at temperatures as low as 210.degree. F.
In terms of processing techniques, it is known to extrude thermoset materials by first B-staging the thermoset materials by heating them prior to extrusion. The B-staged thermoset materials emulate thermoplastic materials and are extrudable to a limited extent. However, the high viscosity of B-staged thermoset materials prevents incorporation of a high percentage of microsphere fillers since the heat and friction developed during the mechanical mixing of the B-staged resin causes the microspheres to be crushed and would limit the weight savings sought to be realized by the incorporation of lightweight microsphere fillers.
Prior art plastic reinforcements fail to provide a lightweight yet strong reinforcement which is thermally stable and competitive in cost to other types of structural reinforcements. These and other problems and disadvantages are overcome by the present invention as summarized below.
Referring now to FIG. 7, the method of making filled thermoset plastic pellets incorporated in the present invention is illustrated in a block diagram. The first step is shown to comprise mixing thermoset resin and microspheres. The mixture is cold extruded, preferably through a plurality of extrusion ports, to form at least one continuous strand which is deposited on an endless belt conveyor. The conveyor preferably moves at a speed substantially equal to the rate that the strand is extruded. The strand is a paste form mixture when extruded which is gelled on the conveyor to form a solid strand without B-staging the material. The strand is then chopped or otherwise formed into pellets. The pellets may be formed into structural reinforcements and cured in place in the structural members as described in Applicant's copending application Ser. No. 811,041, filed Dec. 19, 1985, the disclosure of which is hereby incorporated by reference.
The pellets are preferably coated with an adhesive prior to forming the pellets into structural reinforcements. The thermoset resin in the pellets is preferably a low viscosity epoxy or polyester resin and the microspheres may include expanded or unexpanded microspheres of organic materials or glass. Maximum strength and weight savings can be achieved by combining the resin with microspheres in the range of ratios of 1:2.75 to 3.5 parts by volume.
The mixing steps are performed at ambient temperatures or more preferably at a slightly elevated temperature below the B-staged temperature of the mixture. The mixture is preferably not B-staged in the mixer but is instead heated on the conveyor to a point above B-stage for a time period insufficient to B-stage wherein the strands gel within a very short period of time as they are conveyed to the pelletizer.
The pellets are intended to be used in according to the present invention in a passive restraint for automobile applications.
The pellets comprise uncured thermoset plastic resin which is intermixed with expanded microspheres and converted to its gelled, uncured, solid form. The pellets also preferably include thermally expandable microspheres which permit further reduction of the bulk density of the pellets. Alternatively, the pellet mixture may include a blowing agent which permits expansion of the pellets upon heating.
The pellets are preferably formed of a low viscosity thermoset resin such as a polyester resin or thermally cured epoxy resin. The pellets formed according to the process of the present invention are unique in that they are made by an extrusion process which provides significant processing efficiencies and yet are uncured solid members permitting the passive restraint systems formed with the pellets to be cured during later processing steps. Since the pellets are formed of thermoset resins, they are significantly stronger in terms of tensile and compressive strength as compared to thermoplastic pellets.
The pellets are generally composed of the following constituents in the following approximate ranges:
TABLE I ______________________________________ Constituent Range ______________________________________ Thermoset Resin 100% resin weight Organic or Inorganic Pre- Expanded Microspheres 15-35% resin weight Unexpanded Microspheres 0-10% resin weight Curing Agent 0-3% resin weight Wetting Agent 0-15% resin weight ______________________________________
The cure agent amount stated above would be appropriate for polyester or vinylester systems. If a one-part epoxy resin is used, the cure agent is advantageously in the range of 1 to 10% resin weight, and if a two-part epoxy resin is used, the cure agent is advantageously in the range of 0 to 50% resin weight. As is well known in the industry, the quantity of curing agent depends upon resin and cure system.
The apparatus for making the thermoset pellets of the present invention includes a batch mixer and extruder, or kneader-extruder, wherein the thermoset plastic resin, microspheres and other constituents are combined and from which the mixture is extruded. The mixture is extruded on an endless belt conveyor in paste form as a continuous strand. The endless belt conveyor is substantially synchronously operated with the rate of extrusion since the strand has only limited compressive and tensile strength at the time it is extruded. If the endless belt conveyor were to run too quickly, the strand would be stretched or broken and if ran too slowly, the strand would accumulate on the conveyor. It is preferred to provide an extrusion die having a plurality of linear bores which are oriented at an acute angle, preferably less than 30.degree., relative to the top surface of the conveyor belt. The strength of the strand is increased as the mixture gels. Gelling can be accelerated by exposing the strand to a catalyzing environment, preferably under an infrared heater as it passes along the conveyor. The strands are heated to above their B-stage temperature on the conveyor for a time period less than that required to B-stage the mixture. After gelling, the strands are tack-free, hardened, ductile but substantially uncured. The strands are then conveyed to an unloading station wherein the strands are broken or cut into pellets.
The pellets are coated with an adhesive by a tumbling process. The adhesive is comprised of a thermoset resin having two percent or less tensile elongation. The resin coating is provided primarily to provide adhesion between pellets and to the frame of the passive restraint system. If the coating has low tensile elongation characteristics, the ultimate compression strength of the pellets and passive restraint made with the pellets will be enhanced. Also, the resin coating can improve moisture resistance of the pellets and passive restrains made thereby and assures good long-term strength.
The primary advantage of the method and apparatus used in the present invention is that a simple and continuous process may be used to form highly filled, high strength thermoset pellets which are then usable as a constituent in the manufacture of pre-cast passive restraint systems. In addition, the product made according to the process is superior to thermoplastic pellets in terms of strength and temperature resistance. The pellets made according to the process are also superior to prior art B-staged thermoset pellets because they can be highly filled with unexpanded and expanded microspheres.
The primary advantage of the pellets is their high strength, ultra-low weight and low cost. Also, the thermoset materials have a higher temperature resistance as compared to prior art thermoplastic pellets. Bulk density of the pellets formed according to the present invention may be as low as 12 pounds per cubic foot when no unexpanded microspheres are used and may be as low as approximately 9 pounds per cubic foot if thermally expanded microspheres are included in the pellet mixture. The pellets are solid, that is, they do not contain voids or openings for the purpose of reducing bulk density.
As is well known, thermoset materials generally have greater compressive and tensile strength than thermoplastic materials. When combining this strength advantage with the above density levels, it will be readily appreciated that an extremely high strength and lightweight material is provided. Such a high strength, low density material is ideal for use in automotive applications where weight savings are important.
These pellets are utilized in a knee blocker that is positioned in the area of the instrument panel of an automobile that is under the steering column forward of the knees. The construction of this knee blocker basically consists of a one piece metal stamping which is approximately 12 inches high and runs the full width of the car from hinge pillar to hinge pillar and has a local pocket around the steering column in front of the driver to receive the knee blocker. The other side of the lower instrument panel consists of 1/4 inch thick urethane foam with a vinyl skin attached thereto. The knee blocker is actually sandwiched between the metal stamping and the foam vinyl skin. This pre-cast element is made by vacuum foaming a female pocket and inserting the cast pre-coated pellets into the female pocket. These pellets will act as a crushable medium in that when the knee impacts the lower instrument panel, energy is absorbed by the pulverization and crush of the media.
Furthermore, this pre-cast element can be attached to the metal panel for initial product assurance by means of a variety of methods. First, the pre-cast part can be locked or trapped into the sheet metal at the top by a notch in the sheet metal. It can also be bonded to the metal stamping by means of a butyl tape or adhesive. By a third means of attachment, the vacuum form sheet can be stapled to the metal stamping.
Another advantage of the method of the present invention is that the pellets can conceivably be made from any type of thermoset material.
Other objects, advantages and efficiencies of the present invention will become apparent upon reviewing the attached drawings in view of the following specification and appended claims.